Leading operators and vendors in the Next Generation Mobile Network (NGMN) Alliance are expecting various applications and services to be provided by the fifth generation (5G) network. The 5G network, also been named NR (New Radio) will support a huge amount of applications and services having different performance attributes from delay-sensitive video applications to ultra-low latency real-time applications, from entertainment applications in high-speed vehicles to mobility on demand applications for connected objects, and from best-effort applications to reliable or ultra-reliable applications such as health and security.
In the 4G, namely LTE (Long Term Evolution), network, HARQ (hybrid automatic repeat request) is employed for error detection and correction. In a standard ARQ (automatic repeat request) method, error detection bits are added to data to be transmitted. In Hybrid ARQ, error correction bits are also added. When the receiver receives a data transmission, the receiver uses the error detection bits to determine if data has been lost. If it has, then the receiver may be able to decode and use the error correction bits for recovering the lost data. If the receiver is not able to recover the lost data using the error correction bits, then the receiver may use a second transmission of additional data, including more error correction information, to recover the data. Error correction can be performed by combining information from the initial transmission with additional information from one or more subsequent retransmissions.
FIG. 1 shows an illustration for procedure and soft buffer utility of HARQ. As shown, When UE (user equipment, such as smartphones) receives data from eNB (evolved Node B, namely base station), it will check which HARQ process the data block that is assigned for the data block and then put the received soft information in corresponding soft buffer, which is a part of memory in user equipment. If the data is decoded successfully based on the soft information, the UE will submit the data to upper layer and the soft information is not useful anymore. Otherwise, the soft information should be kept in the soft buffer to wait for combining with that from a retransmission.
Note that in LTE systems, typically each HARQ process has one specific soft buffer of fixed size. However, NR systems which may operate on high frequency bands, are emerging as a promising technology for meeting the exploding bandwidth requirements by enabling multi-Gbps speeds. At such high frequencies, wider channel bandwidth (e.g. 1 GHz) will be the case and thus larger transmission block size needs to be supported.
For NR systems with wider bandwidth, the size of maximum transport block will be much larger than that for LTE. To make simple calculation, the maximum channel bandwidth for LTE is 20 MHz. However, for NR this value may become 2 GHz, which means the required soft buffer size for each HARQ process may be 100 times as LTE if the same TTI length and numerology are used. Although NR may use shorter TTI and larger subcarrier space, the required soft buffer in UE side for one HARQ process is still significantly larger than that of LTE. As we know, soft buffer memory needs high speed Input Output (IO) capability and a large increase of soft buffer means a clear increase of UE cost. Furthermore, taking into possible carrier aggregation possibility, multi-subframe scheduling or multiple connectivity situations, the required soft buffer size for UEs in NR network becomes significantly larger which is not always acceptable.
Hence in NR network, it may be the case that there is not sufficient soft buffer, then efficient utility of the UE soft buffer shall be considered.